The present invention relates to novel antagonists of endothelin useful as pharmaceutical agents, to methods for their production, to pharmaceutical compositions which include these compounds and a pharmaceutically acceptable carrier, and to pharmaceutical methods of treatment. More particularly, the compounds of the present invention are antagonists of endothelin useful in treating elevated levels of endothelin, acute and chronic renal failure, hypertension, myocardial infarction and myocardial ischemia, cerebral vasospasm, cirrhosis, septic shock, congestive heart failure, endotoxic shock, subarachnoid hemorrhage, arrhythmias, asthma, preeclampsia, atherosclerotic disorders including Raynaud's disease and restenosis, angina, cancer, pulmonary hypertension, ischemic disease, gastric mucosal damage, hemorrhagic shock, ischemic bowel disease, and diabetes.
Also, the compounds will be useful in cerebral ischemia or cerebral infarction resulting from a range of conditions such as thromboembolic or hemorrhagic stroke, cerebral vasospasm, head injury, hypoglycemia, cardiac arrest, status epilepticus, perinatal asphyxia, anoxia such as from drowning, pulmonary surgery, and cerebral trauma.
Endothelin is involved in many human disease states.
Several studies have been reported with both peptide and non-peptide ET antagonists showing efficacy in various models of subarachnoid hemorrhage (SAH). For example, BQ-123-prevents early cerebral vasospasm following SAH in various rat (Clozel M., et al., Life Sci., 1993;52:825) and rabbit (Lee K. S., et al., Cerebral Vasospasm, 1993:217; and Neurosurgery, 1994; 34:108) models. FR 139317 significantly inhibited the vasoconstriction of the basilar artery after 7 days in a canine two-hemorrhage model of SAH (Nirei H., et al., Life Sci., 1993;52:1869). BQ-485 also significantly inhibited the vasoconstriction of the basilar artery after 7 days in a canine two-hemorrhage model of SAH (Yano, et al., Biochem Biophys. Res Commun., 1993; 195:969). Ro 46-2005 (Clozel M., et al., Nature, 1993;365:759) has been shown to prevent early cerebral vasospasm following SAH in the rat with no significant effect on systemic arterial blood pressure. Treatment with Ro 47-0203=Bosentan (Clozel, et al., Circulation, 1993;88(4) part 2:0907) to rabbits with SAH had a 36.+-.7% reduction of basilar artery cross-sectional area compared to sham rabbits. All of these studies show in vivo efficacy of endothelin antagonists in cerebral vasospasm resulting from SAH.
Endothelin-1 (ET-1), a potent vasoconstrictor, is a 21 amino acid bicyclic peptide that was first isolated from cultured porcine aortic endothelial cells. Endothelin-1, is one of a family of structurally similar bicyclic peptides which include; ET-2, ET-3, vasoactive intestinal contractor (VIC), and the sarafotoxins (SRTXs).
Several in vivo studies with ET antibodies have been reported in disease models. Left coronary artery ligation and reperfusion to induce myocardial infarction in the rat heart, caused a 4- to 7-fold increase in endogenous endothelin levels. Administration of ET antibody was reported to reduce the size of the infarction in a dose-dependent manner (Watanabe T., et al., "Endothelin in Myocardial Infarction," Nature, (Lond.) 1990;344:114). Thus, ET may be involved in the pathogenesis of congestive heart failure and myocardial ischemia (Margulies K. B., et al., "Increased Endothelin in Experimental Heart Failure," Circulation, 1990;82:2226).
Studies by Kon and colleagues using anti-ET antibodies in an ischemic kidney model, to deactivate endogenous ET, indicated the peptide's involvement in acute renal ischemic injury (Kon V., et al., "Glomerular Actions of Endothelin In Vivo," J. Clin. Invest., 1989;83:1762). In isolated kidneys, preexposed to specific antiendothelin antibody and then challenged with cyclosporine, the renal perfusate flow and glomerular filtration rate increased, while renal resistance decreased as compared with isolated kidneys preexposed to a nonimmunized rabbit serum. The effectiveness and specificity of the anti-ET antibody were confirmed by its capacity to prevent renal deterioration caused by a single bolus dose (150 pmol) of synthetic ET, but not by infusion of angiotensin II, norepinephrine, or the thromboxane A.sub.2 mimetic U-46619 in isolated kidneys (Perico N., et al., "Endothelin Mediates the Renal Vasoconstriction Induced by Cyclosporine in the Rat," J. Am. Soc. Nephrol., 1990;1:76).
Others have reported inhibition of ET-1 or ET-2-induced vasoconstriction in rat isolated thoracic aorta using a monoclonal antibody to ET-1 (Koshi T., et al., "Inhibition of Endothelin (ET)-l and ET-2-Induced Vasoconstriction by Anti-ET-1 Monoclonal Antibody," Chem. Pharm. Bull., 1991;39:1295).
Combined administration of ET-1 and ET-1 antibody to rabbits showed significant inhibition of the blood pressure (BP) and renal blood flow responses (Miyamori I., et al., Systemic and Regional Effects of Endothelin in Rabbits: Effects of Endothelin Antibody," Clin. Exp. Pharmacol. Physiol., 1990;17:691).
Other investigators have reported that infusion of ET-specific antibodies into spontaneously hypertensive rats (SHR) decreased mean arterial pressure (MAP), and increased glomerular filtration rate and renal blood flow. In the control study with normotensive Wistar-Kyoto rats (WKY) there were no significant changes in these parameters (Ohno A., Effects of Endothelin-Specific Antibodies and Endothelin in Spontaneously Hypertensive Rats," J. Tokyo Women's Med. Coll., 1991;61:951).
In addition, elevated levels of endothelin have been reported in several disease states (see Table I below).
Burnett and co-workers recently demonstrated that exogenous infusion of ET (2.5 ng/kg/mL) to anesthetized dogs, producing a doubling of the circulating concentration, did have biological actions (Lerman A., et al., "Endothelin has Biological Actions at Patho-physiological Concentrations," Circulation, 1991; 83:1808). Thus heart rate and cardiac output decreased in association with increased renal and systemic vascular resistances and antinatriuresis. These studies support a role for endothelin in the regulation of cardiovascular, renal, and endocrine function.
In congestive heart failure in dogs and humans, a significant 2- to 3-fold elevation of circulating ET levels has been reported (Rodeheffer R. J., et al., "Circulating Plasma Endothelin Correlates With the Severity of Congestive Heart Failure in Humans," Am. J. Hypertension, 1991;4:9A).
The distribution of the two cloned receptor subtypes, termed ET.sub.A and ET.sub.B, have been studied extensively (Arai H., et al., Nature, 1990;348:730, Sakurai T., et al., Nature, 1990;348:732). The ET.sub.A, or vascular smooth muscle receptor, is widely distributed in cardiovascular tissues and in certain regions of the brain (Lin H. Y., et al., Proc. Natl. Acad. Sci., 1991;88:3185). The ET.sub.B receptor, originally cloned from rat lung, has been found in rat cerebellum and in endothelial cells, although it is not known if the ET.sub.B receptors are the same from these sources. The human ET receptor subtypes have been cloned and expressed (Sakamoto A., et al., Biochem. Biophys. Res. Chem., 1991;178:656, Hosoda K., et al., FEBS Lett., 1991;287:23). The ET.sub.A receptor clearly mediates vasoconstriction and there have been a few reports implicating the ET.sub.B receptor in the initial vasodilatory response to ET (Takayanagi R., et al., FEBS Lett., 1991;282:103). However, recent data has shown that the ET.sub.B receptor can also mediate vasoconstriction in some tissue beds (Panek R. L., et al., Biochem. Biophys. Res. Commun., 1992;183(2):566).
A recent study showed that selective ET.sub.B agonists caused only vasodilation in the rat aortic ring, possibly through the release of EDRF from the endothelium (ibid). Thus, reported selective ET.sub.B agonists, for example, the linear analog ET[1,3,11,15-Ala] and truncated analogs ET[6-21, 1,3,11,15-Ala], ET[8-21,11,15-Ala], and N-Acetyl-ET[10-21,11,15-Ala] caused vasorelaxation in isolated, endothelium-intact porcine pulmonary arteries (Saeki T., et al., Biochem. Biophys. Res. Commun., 1991;179:286). However, some ET analogs are potent vasoconstrictors in the rabbit pulmonary artery, a tissue that appears to possess an ET.sub.B, nonselective type of receptor (ibid).
Plasma endothelin-1 levels were dramatically increased in a patient with malignant hemangioendothelioma (Nakagawa K. et al., Nippon Hifuka Gakkai Zasshi, 1990;100:1453-1456).
The ET receptor antagonist BQ-123 has been shown to block ET-1 induced bronchoconstriction and tracheal smooth muscle contraction in allergic sheep providing evidence for expected efficacy in bronchopulmonary diseases such as asthma (Noguchi, et al., Am. Rev. Respir. Dis., 1992;145(4 Part 2):A858).
Circulating endothelin levels are elevated in women with preeclampsia and correlate closely with serum uric acid levels and measures of renal dysfunction. These observations indicate a role for ET in renal constriction in preeclampsia (Clark B. A., et al., Am. J. Obstet. Gynecol., 1992;166:962-968).
Plasma immunoreactive endothelin-1 concentrations are elevated in patients with sepsis and correlate with the degree of illness and depression of cardiac output (Pittett J., et al., Ann Surg., 1991;213(3):262).
In addition the ET-1 antagonist BQ-123 has been evaluated in a mouse model of endotoxic shock. This ET.sub.A antagonist significantly increased the survival rate in this model (Toshiaki M., et al., 20.12.90. EP 0 436 189 Al).
Endothelin is a potent agonist in the liver eliciting both sustained vasoconstriction of the hepatic vasculature and a significant increase in hepatic glucose output (Gandhi C. B., et al., Journal of Biological Chemistry, 1990;265(29):17432). In addition increased levels of plasma ET-1 have been observed in microalbuminuric insulin-dependent diabetes mellitus patients indicating a role for ET in endocrine disorders such as diabetes (Collier A., et al., Diabetes Care, 1992;15(8):1038).
ET.sub.A antagonist receptor blockade has been found to produce an antihypertensive effect in normal to low renin models of hypertension with a time course similar to the inhibition of ET-1 pressor responses (Basil M. K., et al., J. Hypertension, 1992;10(Suppl 4): S49). The endothelins have been shown to be arrhythmogenic, and to have positive chronotropic and inotropic effects, thus ET receptor blockade would be expected to be useful in arrhythmia and other cardiovascular disorders (Han S. -P., et al., Life Sci., 1990;46:767).
The widespread localization of the endothelins and their receptors in the central nervous system and cerebrovascular circulation has been described (Nikolov R. K., et al., Drugs of Today, 1992;28(5): 303-310). Intracerebroventricular administration of ET-1 in rats has been shown to evoke several behavioral effects. These factors strongly suggest a role for the ETs in neurological disorders. The potent vasoconstrictor action of ETs on isolated cerebral arterioles suggests the importance of these peptides in the regulation of cerebrovascular tone. Increased ET levels have been reported in some CNS disorders, i.e., in the CSF of patients with subarachnoid hemorrhage and in the plasma of women with preeclampsia. Stimulation with ET-3 under conditions of hypoglycemia have been shown to accelerate the development of striatal damage as a result of an influx of extracellular calcium. Circulating or locally produced ET has been suggested to contribute to regulation of brain fluid balance through effects on the choroid plexus and CSF production. ET-1 induced lesion development in a new model of local ischemia in the brain has been described.
Circulating and tissue endothelin immunoreactivity is increased more than twofold in patients with advanced atherosclerosis (Lerman A., et al., New England J. Med., 1991;325:997-1001). Increased endothelin immunoreactivity has also been associated with Buerger's disease (Kanno K., et al., J. Amer. Med. Assoc., 1990;264:2868) and Raynaud's phenomenon (Zamora M. R., et al., Lancet, 1990;336:1144-1147).
An increase of circulating endothelin levels was observed in patients that underwent percutaneous transluminal coronary angioplasty (PTCA) (Tahara A., et al., Metab. Clin. Exp., 1991;40:1235-1237.
Increased plasma levels of endothelin have been measured in rats and humans (Stewart D. J., et al., Ann. Internal Medicine, 1991;114:464-469) with pulmonary hypertension.
Elevated levels of endothelin have also been measured in patients suffering from ischemic heart disease (Yasuda M., et al., Amer. Heart J., 1990;119:801-806) and either stable or unstable angina (Stewart J. T., et al., Br. Heart J., 1991;66:7-9).
Infusion of an endothelin antibody 1 hour prior to and 1 hour after a 60 minute period of renal ischaemia resulted in changes in renal function versus control. In addition, an increase in glomerular platelet-activating factor was attributed to endothelin (Lopez-Farre A., et al., J. Physiology, 1991;444:513-522). In patients with chronic renal failure as well as in patients on regular hemodialysis treatment mean plasma endothelin levels were significantly increased (Stockenhuber F., et al., Clin. Sci. (Lond.), 1992;82:255-258).
Local intra-arterial administration of endothelin has been shown to induce small intestinal mucosal damage in rats in a dose-dependent manner (Mirua S., et al., Digestion, 1991;48:163-172). Furthermore, it has been shown that an anti-ET-1 antibody reduced ethanol-induced vasoconstriction in a concentration-dependent manner (Masuda E., et al., Am. J. Physiol., 1992;262:G785-G790). Elevated endothelin levels have been observed in patients suffering from Crohn's disease and ulcerative colitis (Murch S. H., et al., Lancet, 1992;339:381-384).
At the 3rd International Conference on Endothelin, Houston, Tex., February 1993, the nonpeptide endothelin antagonist RO 46-2005 has been reported to be effective in models of acute renal ischemia and subarachnoid hemorrhage in rats (Clozel M., et al., "Pathophysiological role of endothelin revealed by the first orally active endothelin receptor antagonist," Nature, 1993;365:759). In addition, the ET.sub.A antagonist BQ-123 has been shown to prevent early cerebral vasospasm following subarachnoid hemorrhage (Clozel M. and Watanabe H., Life Sci., 1993;52:825-834.
Recently, an ET.sub.A selective antagonist demonstrated an oral antihypertensive effect (Stein P. D., et al., "The Discovery of Sulfonamide Endothelin Antagonists and the Development of the Orally Active ET.sub.A Antagonist 5-(Dimethylamino)-N-(3,4-dimethyl-5-isoxazolyl)-1-naphthalenesulfonamide," J. Med. Chem., 1994;37:329-331.
Table I below summarizes some of the conditions in which ET-l is involved.
TABLE I ______________________________________ Plasma Concentrations of ET-1 in Humans ET Plasma Levels Normal Reported Condition Control (pg/mL) ______________________________________ Atherosclerosis 1.4 3.2 pmol/L Surgical operation 1.5 7.3 Buerger's disease 1.6 4.8 Takayasu's arteritis 1.6 5.3 Cardiogenic shock 0.3 3.7 Congestive heart failure (CHF) 9.7 20.4 Mild CHF 7.1 11.1 Severe CHF 7.1 13.8 Dilated cardiomyopathy 1.6 7.1 Preeclampsia 10.4 pmol/L 22.6 pmol/L Pulmonary hypertension 1.45 3.5 Acute myocardial infarction 1.5 3.3 (several reports) 6.0 11.0 0.76 4.95 0.50 3.8 Subarachnoid hemorrhage 0.4 2.2 Ulcerative colitis 0-24 fmol/mg 20-50 fmol/mg Cold pressor test 1.2 8.4 Raynaud's phenomenon 1.7 5.3 Raynaud's/hand cooling 2.8 5.0 Hemodialysis &lt;7 10.9 (several reports) 1.88 4.59 Chronic renal failure 1.88 10.1 Acute renal failure 1.5 10.4 Uremia before hemodialysis 0.96 1.49 Uremia after hemodialysis 0.96 2.19 Essential hypertension 18.5 33.9 Sepsis syndrome 6.1 19.9 Postoperative cardiac 6.1 11.9 Inflammatory arthritides 1.5 4.2 Malignant hemangioendothelioma 4.3 16.2 (after removal) ______________________________________
Copending U.S. application Ser. No. 08/384,083 covers nonpeptide endothelin antagonists of formula ##STR1## or a tautomeric open chain keto-acid form thereof or a pharmaceutically acceptable salt thereof wherein
R.sub.1 is cycloalkyl substituted or unsubstituted of from 3 to 12 carbon atoms, PA1 R.sub.2 is alkyl substituted or unsubstituted straight, or branched of from 1 to 12 carbon atoms, PA1 R.sub.3 is alkyl substituted or unsubstituted straight, or branched, of from 1 to 12 carbon atoms, PA1 R.sub.4 is hydroxy or OR.sub.5, PA1 X is O or S; PA1 R.sup.2 represents a hydrogen atom or hydroxyl group, PA1 R.sup.3 and R.sup.4 each represent a lower alkyl group, or PA1 R.sup.3 and R.sup.4 together represent an alkylene group with a total of 3 to 6 carbons. The compounds are disclosed as insect repellents. PA1 R.sub.2 is alkyl substituted or unsubstituted straight, or branched, of from 1 to 12 carbon atoms, PA1 R.sub.3 is alkyl substituted or unsubstituted straight, or branched, of from 1 to 12 carbon atoms, PA1 R.sub.4 is alkyl, unsubstituted or substituted with from 1 to 5 substituents; PA1 X is O or S; and PA1 Y is absent or NH. This application is hereby incorporated by reference. PA1 R.sub.2 is alkyl substituted or unsubstituted straight, or branched, of from 1 to 12 carbon atoms, PA1 R.sub.3 is alkyl substituted or unsubstituted straight, or branched, of from 1 to 12 carbon atoms, PA1 R is phenyl substituted with from 1 to 5 substituents; PA1 R.sub.2 is alkyl substituted or unsubstituted straight, or branched, of from 1 to 7 carbon atoms; and PA1 R.sub.3 is aryl substituted or unsubstituted, PA1 R.sub.1 is 4-piperonyl, PA1 R.sub.3 is phenyl, PA1 3-Benzo[1,3]dioxol-5-yl-4-(4-methoxy-3-methylbenzyl)-5-(4-methoxyphenyl)-5H -furan-2-one; PA1 3-Benzo[1,3]dioxol-5-yl-4-(4-methoxybenzyl)-5-(3,4,5-trimethoxyphenyl)-5H-f uran-2-one; PA1 3-Benzo[1,3]dioxol-5-yl-5-(4-methoxyphenyl)-4-(3,4,5-trimethoxybenzyl)-5H-f uran-2-one; PA1 3-Benzo[1,3]dioxol-5-yl-4-benzyl-5-(4-methoxyphenyl)-5H-furan-2-one; PA1 3-Benzo[1,3]dioxol-5-yl-4-(3-dimethylaminobenzyl-5-(4-methoxyphenyl)-5H-fur an-2-one; PA1 3-Benzo[1,3]dioxol-5-yl-4-(3,4-dimethyoxy-5-(4-carbethoxybutyloxy)benzyl)-5 -(4-methoxyphenyl)-5H-furan-2-one; PA1 3-Benzo[1,3]dioxol-5-yl-4-(3,4,5-trimethoxybenzyl)-5-(4-ethoxy-3-methylphen yl)-5H-furan-2-one; PA1 3-(5-Methoxy)benzo[1,3]dioxol-5-yl-4-(3,4,5-trimethoxybenzyl)-5-(4-methoxyp henyl)-5H-furan-2-one; PA1 3-3,5-Dimethylphenyl-4-(benzyl)-5-(4-methoxyphenyl)-5H-furan-2-one; PA1 3-3,5-Dimethylphenyl-4-(3,4,5-trimethoxybenzyl)-5-(4-methoxyphenyl)-5H-fura n-2-one; PA1 3--(5-Methoxy)benzo[1,3]dioxol-5-yl-4-(4-methoxy-3-methylbenzyl)-5-(4-metho xyphenyl)-5H-furan-2-one; PA1 3--(5-Methoxy)benzo[1,3]dioxol-5-yl-4-(benzyl)-5-(4-methoxyphenyl)-5H-furan -2-one; PA1 3-- (5-Methoxy)benzo[1, 3]dioxol-5-yl-4- (3,4,5-trimethoxybenzyl) -5- (4-methoxyphenyl) -5H-furan-2-one; PA1 3-Benzo[1,3]dioxol-5-yl-4- (4-methoxy-3-methylbenzyl) -5-(2-(7,8,9,10-tetrahydro) -naphthyl) -5H-furan-2-one; and PA1 3-Benzo[1,3]dioxol-5-yl-4-(3,4,5-trimethoxybenzyl-5-(3,4-dimethoxyphenyl)-5 H-furan-2-one.
phenyl substituted with from 1 to 5 substituents, PA2 naphthyl unsubstituted or substituted with from 1 to 5 substituents, or PA2 heteroaryl unsubstituted or substituted with from 1 to 5 substituents; PA2 cycloalkyl substituted or unsubstituted of from 3 to 12 carbon atoms, PA2 aryl which is unsubstituted or substituted with from 1 to 5 substituents, PA2 heteroaryl which is unsubstituted or substituted with from 1 to 3 substituents; PA2 cycloalkyl substituted or unsubstituted of from 3 to 12 carbon atoms, PA2 aryl which is unsubstituted or substituted with from 1 to 5 substituents, PA2 heteroaryl which is unsubstituted or substituted with from 1 to 3 substituents; PA2 SR.sub.5, wherein R.sub.5 is alkyl or substituted alkyl of from 1 to 7 carbon atoms, or PA2 (CH.sub.2).sub.n OR.sub.5 wherein n is an integer of from 1 to 3; PA2 phenyl substituted with from 1 to 5 substituents, PA2 naphthyl unsubstituted or substituted with from 1 to 5 substituents, or PA2 heteroaryl unsubstituted or substituted with from 1 to 5 substituents; PA2 cycloalkyl substituted or unsubstituted of from 3 to 12 carbon atoms, PA2 aryl which is unsubstituted or substituted with from 1 to 5 substituents, PA2 heteroaryl which is unsubstituted or substituted with from 1 to 5 substituents, PA2 cycloalkyl substituted or unsubstituted of from 3 to 12 carbon atoms, PA2 aryl which is unsubstituted or substituted with from 1 to 5 substituents, PA2 heteroaryl which is unsubstituted or substituted with from 1 to 5 substituents; PA2 aryl unsubstituted or substituted with from 1 to 5 substituents; PA2 heteroaryl unsubstituted or substituted with from 1 to 5 substituents; PA2 phenyl substituted with from 1 to 5 substituents, PA2 naphthyl unsubstituted or substituted with from 1 to 5 substituents, or PA2 heteroaryl unsubstituted or substituted with from 1 to 5 substituents; PA2 cycloalkyl substituted or unsubstituted of from 3 to 12 carbon atoms; and PA2 cycloalkyl substituted or unsubstituted of from 3 to 12 carbon atoms, PA2 aryl which is unsubstituted or substituted with from 1 to 5 substituents, PA2 heteroaryl which is unsubstituted or substituted with from 1 to 5 substituents. PA2 heteroaryl substituted or unsubstituted. PA2 3-methoxyphenyl, PA2 3,5 dimethyl, PA2 3,5-dimethoxyphenyl, ##STR5## 3-methoxy-4,5-methylenedioxyphenyl; R.sub.2 is benzyl, PA2 4-piperonylmethyl, PA2 4-isopropylbenzyl, PA2 1-naphthylmethyl, PA2 2-naphthylmethyl, PA2 3-thiophenylmethyl, PA2 2-thiophenylmethyl, PA2 3,4-dichlorobenzyl, PA2 3(N-Me)indolylmethyl, PA2 3,4-dimethoxybenzyl, PA2 4-Me.sub.2 aminobenzyl, PA2 3-Me.sub.2 aminobenzyl, PA2 4-isopropylbenzyl, PA2 4-chlorobenzyl, PA2 4-methoxybenzyl, PA2 4-methylbenzyl, PA2 3-methylbenzyl, PA2 4-isopropoxybenzyl, PA2 4-acetamidobenzyl, PA2 4-methylsulfonylbenzyl, PA2 3-methyl-4-methoxybenzyl, PA2 3-allyloxy-4-methoxybenzyl, PA2 3,4,5-trimethoxybenzyl, PA2 3-n-propoxybenzyl, PA2 4-thiomethylbenzyl, PA2 3-carbethoxybenzyl, PA2 4-carbethoxybenzyl, PA2 3-methoxybenzyl, PA2 2-methoxybenzyl, PA2 3-chlorobenzyl, or PA2 cyclohexylmethyl; PA2 4-methylphenyl, PA2 4-methoxyphenyl, ##STR6## 3-methoxyphenyl, 3-methyl-4-methoxyphenyl, PA2 3,4-dimethoxyphenyl, or PA2 2,4-dimethoxyphenyl.
with the proviso that when R.sub.1 is monosubstituted phenyl and the substituent is p-methoxy, R.sub.3 is not unsubstituted phenyl, monosubstituted phenyl, or mesityl and with the further proviso when R.sub.2 is alkyl substituted, the substituent is not oxygen at the .alpha.-position to the furanone ring.
This application for patent is hereby incorporated by reference.
Japanese Patent Application Number 5[1993]178706 covers compounds of formula ##STR2## where R.sup.1 represents a 1 to 10 carbon alkyl group, 3 to 6 carbon cycloalkyl group, 2 to 10 carbon alkenyl group, 2 to 10 carbon alkynyl group, or phenylalkyl group with a total of less than 10 carbons,
U.S. application Ser.No. 60/000,904 covers compounds of Formula I ##STR3## or a pharmaceutically acceptable salt thereof wherein: R.sub.1 is cycloalkyl substituted or unsubstituted of from 3 to 12 carbon atoms,